Osmotic Stress Suppresses Cell Wall Stiffening and the Increase in Cell Wall-Bound Ferulic and Diferulic Acids in Wheat Coleoptiles

The relationship between the mechanical properties of cell walls and the levels of wall-bound ferulic (FA) and diferulic (DFA) acids was investigated in wheat (Triticum aestivum L.) coleoptiles grown under osmotic stress (60 mM polyethylene glycol [PEG] 4000) conditions. The cell walls of stressed coleoptiles remained extensible compared with those of the unstressed ones. The contents of wall-bound FA and DFA increased under unstressed conditions, but the increase was substantially reduced by osmotic stress. In response to PEG removal, these contents increased and reached almost the same levels as those of the unstressed coleoptiles. A close correlation was observed between the contents of FA and DFA and the mechanical properties of cell walls. The activities of phenylalanine ammonia-lyase and tyrosine ammonia-lyase increased rapidly under unstressed conditions. Osmotic stress substantially reduced the increases in enzyme activities. When PEG was removed, however, the enzyme activities increased rapidly. There was a close correlation between the FA levels and enzyme activities. These results suggest that in osmotically stressed wheat coleoptiles, reduced rates of increase in phenylalanine ammonia-lyase and tyrosine ammonia-lyase activities suppress phenylpropanoid biosynthesis, resulting in the reduced level of wall-bound FA that, in turn, probably causes the reduced level of DFA and thereby maintains cell wall extensibility.     

Light-induced increase in the contents of ferulic and diferulic acids in cell walls of Avena coleoptiles: its relationship to growth inhibition by light

White fluorescent light (5 W m⁻²) inhibited Avena coleoptile growth. Light caused in increase in minimum stress relaxation time and a decrease in extensibility (strain/load) of coleoptile cell walls. Light increased the contents of ferulic acid (FA) and diferulic acid (DFA) ester-linked to the hemicellulose I in cell walls. These changes in the phenolic contents correlated with those of the mechanical properties of cell walls, suggesting that light stimulates the formation of DFA in hemicellulose I, making cell walls rigid, and thus results in growth inhibition. The ratio of DFA to FA was almost constant in the dark, but decreased in light, although it was almost constant in Oryza coleoptiles either in the dark or in light (Tan et al. 1992). From this fact, it is speculated that in the light condition, the formation of DFA in cell walls is limited in the step of the peroxidase catalyzed coupling reaction to produce DFA, while in the dark it is limited in the step of the feruloylation of hemicellulose I.     

White light promotes the formation of diferulic acid in maize coleoptile cell walls by enhancing PAL activity

To elucidate the mechanism by which white fluorescent light (5 W m^-2) stimulates the formation of diferulic acid (DFA) in cell walls, the effect of light on phenylalanine-and tyrosine-ammonia-lyase (PAL, EC 4.3.1.5 and TAL, EC 4.3.1.5) and peroxidase activities was studied using coleoptiles of maize ( Zea mays L. cv. Cross Bantam T51). Growth rate of dark-grown coleoptiles was highest at the basal zone and decreased towards the tip, while continuous irradiation caused an inhibition of growth, especially at the basal zone. Light decreased the cell wall extensibility in all zones of the coleoptile. The amounts of DFA, ferulic acid (FA) and p -coumaric acid ( p -CA) increased by severalfold in cell walls of light-grown maize coleoptiles as compared with those grown in the dark. Strong correlations were observed between the increase in the contents of either DFA, FA or p -CA and the decrease in cell wall extensibility. Light decreased the wall-bound peroxidase activity. No correlation was found between DFA content and peroxidase activity. The activities of PAL and TAL were enhanced upon white light irradiation. The increment in either DFA, FA or p -CA content was correlated with an increase in PAL activity, but not with that in TAL activity. White light may promote DFA formation in the cell walls of maize coleoptiles by enhancing PAL activity.     

Modification of chemical properties of cell walls by silicon and its role in regulation of the cell wall extensibility in oat leaves

Effects of silicon on the mechanical and chemical properties of cell walls in the second leaf of oat (Avena sativa L.) seedlings were investigated. The cell wall extensibility in the basal region of the second leaf was considerably higher than that in the middle and subapical regions. Externally applied silicon increased the cell wall extensibility in the basal region, but it did not affect the extensibility in the middle and subapical regions. The amounts of cell wall polysaccharides and phenolic compounds, such as diferulic acid (DFA) and ferulic acid (FA), per unit length were lower in the basal region than in the middle and subapical regions of the leaf, and silicon altered these amounts in the basal region. In this region, silicon decreased the amounts of matrix polymers and cellulose per unit length and of DFA and FA, both per unit length and unit matrix polymer content. Silicon treatment also lowered the activity of phenylalanine ammonia-lyase (PAL, EC 4.3.1.5) in the basal region. In contrast, the amount of silicon in cell walls increased in response to silicon treatment in three regions. These results suggest that in the basal region, silicon reduces the net wall mass and the formation of phenolic acid-mediated cross-linkages between wall polysaccharides. Such modifications of wall architecture may be responsible for the silicon-induced increase in the cell wall extensibility in oat leaves.     

Osmotic Stress-Induced Growth Suppression of Dark-Grown Wheat (Triticum aestivum L.) Coleoptiles

Application of 60 mM polyethylene glycol (PEG) to dark-grownwheat (Triticum aestivum L.) roots substantially reduced growthof coleoptiles. However, when PEG was removed, the growth rateof these coleoptiles greatly increased. Cell walls of stressedcoleoptiles remained loosened as compared with those of unstressedones. The osmotic potential of the stressed coleoptiles decreasedto that of the 60 mM PEG solution. On the other hand, the extentof decrease in the osmotic potential of stressed roots was smallerthan that of stressed coleoptiles. The osmotic potential difference() between the cell sap and the incubation medium of stressedroots was substantially higher than that of unstressed ones.The amount of ink moved from roots, where it was applied, tothe apical region of coleoptiles was significantly reduced underosmotic stress conditions. When water was exogenously appliedto abraded coleoptiles, the growth of these stressed coleoptileswas greatly promoted. These results suggest that inhibitionof coleop-tile growth under osmotic stress conditions is notdirectly related to a decrease in cell wall extensibility orto loss of the capacity to maintain osmotic potential gradients,but is caused by the reduction of the water supply from theroots to the coleoptiles. (Received August 26, 1996; Accepted December 18, 1996)     

Changes in cellular osmotic potential and mechanical properties of cell walls during light-induced inhibition of cell elongation in maize coleoptiles

The growth rate of maize ( Zea mays L. cv. Cross Bantam T51) coleoptiles in the dark was highest at the basal zone and decreased towards the tip. Growth was strongly inhibited by white fluorescent light (5 W m⁻²), especially in the basal zone of coleoptiles. Light irradiation caused an increase in the values of stress-relaxation parameters, the minimum stress-relaxation time and the relaxation rate and a decrease in the extensibility (strain/stress) of the cell walls at all zones. In addition, during growth, the accumulation of osmotic solutes was strongly inhibited by white light irradiation, resulting in an increased osmotic potential. The influences of white light on the mechanical properties of the cell wall and the osmotic potential of the tissue sap were most prominent in the basal zone. Significant correlations were observed between the increment of coleoptile length and the mechanical properties of the cell walls or the osmotic potential of the tissue sap and osmotic solutes content. Furthermore, light inhibited the outward bending of split coleoptile segments. These facts suggest that white light inhibits elongation of maize coleoptiles by modifying both the mechanical properties of the cell walls and cellular osmotic potential, which control the rate of water uptake.     

Abscisic Acid Suppresses the Increases in Cell Wall-Bound Ferulic and Diferulic Acid Levels in Dark-Grown Wheat (Triticum aestivum L.) Coleoptiles

Application of abscisic acid (ABA) to dark-grown wheat (Triticumaestivum L.) roots interfered the cell wall hardening of coleoptilesduring several days of the treatment. Although the amounts ofwall-bound diferulic (DFA) and ferulic (FA) acids in coleoptilesincreased as the coleoptiles grew, ABA substantially reducedtheir increases. When ABA was removed, however, these contentsincreased and reached levels near those of control coleoptiles.A close correlation was observed between the levels of DFA andFA and the mechanical properties of cell walls. The ratio ofthe amount of DFA to FA was almost constant irrespective ofgrowth conditions. The activities of phenylalanine- (PAL) andtyrosine-ammonia-Iyase (TAL) increased rapidly in the controlcoleoptiles. ABA greatly reduced the increases in these enzymeactivities. In response to ABA removal, the enzyme activitiesincreased rapidly. There was a close correlation between theincrease in FA level and the changes in enzyme activities. Theseresults suggest that ABA suppresses the increases in PAL andTAL activities in wheat coleoptiles, resulting in the reducedlevel of wall-bound FA, which, in turn, may cause the reducedDFA level and thereby maintain cell wall extensibility. (Received January 10, 1997; Accepted April 22, 1997)     

Increase in the level of arabinoxylan–hydroxycinnamate network in cell walls of wheat coleoptiles grown under continuous hypergravity conditions

Changes in the amount and composition of cell wall constituents in response to continuous hypergravity stimuli were studied in wheat ( Triticum aestivum L.) coleoptiles. The lengths of coleoptiles grown under hypergravity (300  g ) conditions for 2–4 days from germination stage were 60–70% of those of 1  g control. However, the net amounts of hemicellulosic polysaccharides and cellulose in hypergravity-treated coleoptiles increased progressively as much as those in the control coleoptiles. As a result, their contents per unit length of coleoptile largely increased under hypergravity conditions. In the hemicellulose fraction, the amounts of arabinose and xylose, the major components of the fraction, prominently increased in response to hypergravity. When hemicellulosic polysaccharides were separated into neutral and acidic polymers by an anion-exchange column, the amounts of the acidic fraction consisting of (glucurono)arabinoxylans were higher in hypergravity-treated coleoptiles than in control coleoptiles. The amounts of cell wall-bound ferulic acid and diferulic acid (DFA) increased dramatically in both 1  g control and hypergravity-treated coleoptiles. Particularly, the amounts of DFA in hypergravity-treated coleoptiles were significantly higher than those in control coleoptiles during the incubation period. These results suggest that continuous hypergravity increases the rigid network structures via arabinoxylan–hydroxycinnamate cross-links within cell wall architecture in wheat coleoptiles. These structures may have a load-bearing function and contribute to construct the stable cell wall against the gravitational force.     

Diferulic and ferulic acid in the cell wall of Avena coleoptiles—Their relationships to mechanical properties of the cell wall

Alkaline hydrolysis liberated ferulic and diferulic acid from polysaccharides of the Avena coleoptile ( Avena sativa L. cv. Victory I) cell walls. The amount of the two phenolic acids bound to cell walls increased substantially at day 4–5 after sowing, when the growth rate of the coleoptile started to decrease. The level of these acids was almost constant from the tip to base in 3-day-old coleoptiles, but increased toward the basal zone in 4- and 5-day-old ones. The ratio of diferulic acid to ferulic acid was almost constant irrespective of coleoptile age and zone. An increase in the amount of ferulic and diferulic acids bound to cell wall polysaccharides correlated with a decrease in extensibility and with an increase in minimum stress-relaxation time and relaxation rate of the cell wall. The level of lignin in the cellulose fraction increased as coleoptiles aged, but this increase did not correlate with changes in mechanical properties of the cell walls. These results suggest that ferulic acid, ester-linked to cell wall polysaccharides, is oxidized to give diferulic acid, which makes the cell wall mechanically rigid by cross-linking matrix polysaccharides and results in limited cell extension growth. In addition, it is probable that the step of feruloylation of cell wall polysaccharides is rate-limiting in the formation of in-termolecular bridges by diferulic acid in Avena coleoptile cell walls.     

Cell wall oxalate oxidase modifies the ferulate metabolism in cell walls of wheat shoots

Oxalate oxidase (OXO) utilizes oxalate to generate hydrogen peroxide, and thereby acts as a source of hydrogen peroxide. The present study was carried out to investigate whether apoplastic OXO modifies the metabolism of cell wall-bound ferulates in wheat seedlings. Histochemical staining of OXO showed that cell walls were strongly stained, indicating the presence of OXO activity in shoot walls. When native cell walls prepared from shoots were incubated with oxalate or hydrogen peroxide, the levels of ester-linked diferulic acid (DFA) isomers were significantly increased. On the other hand, the level of ester-linked ferulic acid (FA) was substantially decreased. The decrease in FA level was accounted neither by the increases in DFA levels nor by the release of FA from cell walls during the incubation. After the extraction of ester-linked ferulates, considerable ultraviolet absorption remained in the hemicellulosic and cellulose fractions, which was increased by the treatment with oxalate or hydrogen peroxide. Therefore, a part of FA esters may form tight linkages within cell wall architecture. These results suggest that cell wall OXO is capable of modifying the metabolism of ester-linked ferulates in cell walls of wheat shoots by promoting the peroxidase action via supply of hydrogen peroxide.     

Xyloglucan Endotransglycosylase Activity, Microfibril Orientation and the Profiles of Cell Wall Properties Along Growing Regions of Maize Roots

A series of physical and chemical analyses were made on theexpanding zone of maize seedling roots grown in hydroponics.Comparison of longitudinal profiles of local relative elementalgrowth rate and turgor pressure indicated that cell walls becomelooser in the apical 5 mm and then tighten 5–10 mm fromthe root tip. Immersion of roots in 200 mol m^–3 mannitol(an osmotic stress of 0·48 MPa) rapidly and evenly reducedturgor pressure along the whole growing region. Growth was reducedto a greater extent in the region 5–10 mm from the roottip than in the apical region. This indicated rapid wall-looseningin the root tip, but not in the more basal regions. Following 24 h immersion in 400 mol m^–3 mannitol (an osmoticstress of 0·96 MPa) turgor had recovered to pre-stressedvalues. Under this stress treatment, growth was reduced in theregion 4–10 mm from the root tip, despite the recoveryof turgor, indicating a tightening of the wall. In the rootapex, local relative elemental growth rate was unchanged incomparison to control tissue, showing that wall properties herewere similar to the control values. Cellulose microfibrils on the inner face of cortical cell wallsbecame increasingly more parallel to the root axis along thegrowth profile of both unstressed and stressed roots. Orientationdid not correlate with the wall loosening in the apical regionof unstressed roots, or with the tightening in the region 5–10mm from the root tip following 24 h of osmotic stress. Longitudinal profiles of the possible wall-loosening enzymexyloglucan endotransglycosylase (XET) had good correspondencewith an increase in wall loosening during development. In thezone of wall tightening following osmotic stress, XET activitywas decreased per unit dry weight (compared with the unstressedcontrol), but not per unit fresh weight. Key words: Osmotic stress, turgor, growth, cell wall properties, microfibrils, XET     

Leaf morphology and water status changes in Triticum durum under water stress

Water relations, leaf morphology and the chemical composition of cell walls in irrigated and unirrigated plants of three durum wheat eultivars were measured at two growth stages (booting and flowering). Plant response to water stress differed at the two stages: cell wall elasticity increased at booting and osmotic potential values decreased at flowering; this may be due to the changes in stress history, leaf development and plant growth stage between the two harvests. Leaf tissue characteristics were modified by water stress only at flowering: accumulation of fibrous constituents and hemicellulose in the cell walls, reduction of acid detergent fiber (ADF) per unit of leaf area, increase in specific leaf weight (SLW), decrease in turgid weight/dry weight ratio (TW/DW) and alteration in mesophyll cell morphology (cell area / ceil perimeter ratio) were observed.
Generally, cv. Valforte (the less drought-resistant cultivar) had the greatest mesophyll cell area and perimeter and it had greater values of neutral detergent fiber (NDF) at the booting stage than cv. Appulo. Reactivity to water stress differed in the eultivars: Valforte showed the greatest increase in hemicellulose content and decrease in cell dimensions under drought at flowering.
No significant relationships between osmotic potential and mesophyll cell characters were observed; there were no correlations among cell wall elasticity, cell morphology and the chemical components of leaf tissue. The total fiber content and the hemicellulose per unit of leaf area were correlated with the TW/DW ratio at flowering. This parameter decreased more in plants subjected to water stress owing to accumulation of hemicellulose. Correlations between leaf structural constituents and $$ suggest that the absorptive capacity of the cell wall may significally affect the osmotic volume of the cell.     

Increased Molecular Mass of Hemicellulosic Polysaccharides is Involved in Growth Inhibition of Maize Coleoptiles and Mesocotyls under Hypergravity Conditions

Zea mays L. cv. Cross Bantam T51) coleoptiles and mesocotyls was suppressed by hypergravity at 30 g and above. Acceleration at 300 g significantly decreased the mechanical extensibility of cell walls of both organs. Hypergravity increased the amounts of hemicellulose and cellulose per unit length in mesocotyl walls, but not in coleoptile walls. The weight-average molecular masses of hemicellulosic polysaccharides were also increased by hypergravity in both organs. On the other hand, the activities of β-glucanases extracted from coleoptile and mesocotyl cell walls were decreased by hypergravity. These results suggest that the decreased activities of β-glucanases by hypergravity cause an increase in the molecular mass of hemicellulosic polysaccharides of both organs. The upshift of molecular mass of hemicellulosic polysaccharides as well as the thickening of cell walls under hypergravity conditions seems to be involved in making the cell wall mechanically rigid, thereby inhibiting elongation growth of maize coleoptiles and mesocotyls. Received 22 February 1999/ Accepted in revised form 20 April 1999     

小麦胚芽鞘扩展蛋白特性及对水分胁迫的响应

扩展蛋白是植物细胞壁延伸过程中的关键调节因子,在植物的生长发育以及对逆境的响应过程中起着重要作用。本文选用小麦(HF 9703)胚芽鞘为材料,采用Hepes法和SDS法分别提取小麦胚芽鞘扩展蛋白,通过改良的植物组织伸长测定仪测定其活性,并利用扩展蛋白抗体进行免疫印迹以检测其丰度,主要研究了小麦胚芽鞘扩展蛋白的特性及对水分胁迫的响应。结果表明:Hepes法提取的扩展蛋白活性较高,而SDS法的提取效率高;离体小麦胚芽鞘扩展蛋白的活性具有pH依赖性,且随缓冲液的交替更换(pH 4.5:pH 6.8)而反复逆转;扩展蛋白主要定位于细胞壁中;小麦胚芽鞘扩展蛋白和黄瓜下胚轴扩展蛋白具有交叉重组活性,但这种活性具有种属特异性。水分胁迫诱导小麦胚芽鞘扩展蛋白的活性和丰度提高,扩展蛋白活性的提高在小麦对水分胁迫的抗性方面可能具有重要作用。     

Analysis of the Growth Response of Air-Grown Rice Coleoptiles to Submergence

The effect of submergence of air-grown rice seedlings (Oryza sativa L. var. Sasanishiki) on coleoptile growth and ultrastructure, extensibility and chemical composition of the cell walls was investigated. The lag-time between start of submergence and the onset of the enhancement of growth was less than 4 h. The growth response was associated with a drastic thinning of the cell walls and an increase in wall extensibility. At the outer epidermal wall of both air-grown and submerged coleoptiles electron-dense (osmiophilic) particles were detected. During submergence, the net accumulation of cellulose and hemicellulose was reduced, but the increase in pectic substances was unaffected. Submergence caused an 80% inhibition of the net accumulation of wall-bound phenolics (ferulic- and diferulic acid) compared with air-grown controls. The osmotic concentration of the tissue saps was not affected by submergence. Our results support the hypothesis that rapid coleoptile elongation under water is caused by an inhibition of the formation of phenolic cross-links between matrix polysaccharides via diferulate, which results in a mechanical stiffening of the cell walls in the air-grown coleoptile.     

Phenylalanine ammonia-lyase and cell wall peroxidase are cooperatively involved in the extensive formation of ferulate network in cell walls of developing rice shoots

The relationship between the formation of cell wall-bound ferulic acid (FA) and diferulic acid (DFA) and the change in activities of phenylalanine ammonia-lyase (PAL) and cell wall-bound peroxidase (CW-PRX) was studied in rice shoots. The length and the fresh mass of shoots increased during the growth period from day 4 to 6, while coleoptiles ceased elongation growth on day 5. The amounts of FA and DFA isomers as well as cell wall polysaccharides continued to increase during the whole period. The activities of PAL and CW-PRX greatly increased in the same manner during the period. There were close correlations between the PAL activity and ferulate content or between the CW-PRX activity and DFA content. The expression levels of investigated genes for PAL and putative CW-PRX showed good accordance with the activities of these enzymes. These results suggest that increases in PAL and CW-PRX activities are cooperatively involved in the formation of ferulate network in cell walls of rice shoots and that investigated genes may be, at least in part, associated with the enzyme activities. The substantial increase in such network probably causes the maturation of cell walls and thus the cessation of elongation growth of coleoptiles.     

Mechanical properties and polysaccharide nature of the cell walls of maize root

Changes in mechanical properties and chemical nature of the cell walls of the different zones along elongating maize ( Zea mays L. cv. LG 11) roots were analyzed and the following results were obtained. (1) The apical region 2 to 5 mm from the tip of 15 mm long roots showed rapid elongation whereas the region 8–10 mm from the tip showed very little growth. (2) The minimum stress-relaxation time (To) and the mean stress-relaxation rate (R) of the cell wall were small whereas the maximum stress-relaxation time (Tm) was large in the region where cell elongation was optimum. The To and R increased and the Tm decreased gradually towards the base of the root. (3) The amounts of non-cellulosic polysaccharides of the cell wall were highest in the region 1.5–2.5 mm from the tip, decreasing until 5 mm from the tip, and then increasing towards the base. However, the proportion of this fraction in the total cell wall polysaccharides was highest in the extreme tip (cap and meristem, 0–1 mm) and decreased towards the base. (4) Major neutral sugars constituting the non-cellulosic polysaccharides of the cell wall were xylose, arabinose, galactose and glucose, with minor amounts of rhamnosc. mannose and fucose. The 1–15 mm region was on the whole rich in glucose and xylose and contained arabinose to a lesser extent. However, the chemical nature in the apical region, (0–2 mm, was rather special, being rich in galactose and fucose. (5) The cell wall of maize roots contained, as a whole, only little pectic substances but was high in hemicellulose 1 (rich in xylose, arabinose and glucose) and hemicellulose 2 (rich in glucose and xylose). (6) It appeared that in the elongating region (apical 2 to 5 mm) the cell elongation rate (CET) showed a rather good correlation with the parameters of mechanical properties (To, Tm and R) and with neutral sugar compositions in the non-cellulosic polysaccharides.     

Correlation between cell wall extensibility and the content of diferulic and ferulic acids in cell walls of Oryza sativa coleoptiles grown under water and in air

Rice ( Oryza sativa L. cv. Sasanishiki) coleoptiles grown under water achieved greater length than those grown either in air or under water with constant air bubbling. The extensibility of cell walls in coleoptiles grown under water was larger than that in the other treatments. Per unit length of the coleoptile, the content of ferulic and diferulic acids ester-linked to hemicelluloses was higher in air and bubbling type coleoptiles than in water type ones. The extensibility of the coleoptile cell walls correlated with the content of diferulic acids per unit length and per hemicellulose, suggesting that the enhancement of the formation of diferulic acid bridges in hemicelluloses in air or under water with air bubbling makes the cell walls mechanically rigid; thereby inhibiting cell elongation in rice coleoptiles. In addition, the ratio of diferulic acid to ferulic acid was almost constant irrespective of coleoptile age, zone and growth conditions, suggesting that the feruloylation of hemicelluloses is rate-limiting in the formation of diferulic acid bridges in the cell walls of rice coleoptiles.     

Salt stress enhances proline utilization in the apical region of barley roots

Accumulation of an osmoprotectant, proline, is enhanced in response to salinity in plants. Here, by immunohistochemical analysis, we demonstrated that proline transporter (HvProT) was highly expressed in the apical region of barley roots under salt stress. Free proline was accumulated more in the basal region than in the apical region of barley roots under salt stress, although expression level of HvProT was higher in the apical region. On the other hand, salt stress increased proline and hydroxyproline contents in the cell wall fraction of the root apical region, suggesting increment of proline utilization. Expression of the genes encoding cell wall proteins (proline rich protein and extensin) and cellulose synthase was induced in barley roots by salt stress. These findings indicated that free proline transported by HvProT presumably behaved as a component of cell wall synthesis in the apical region of barley roots under salt stress.     

Roles of auxin and gibberellic acid in growth and maturation of epicotyls of Vigna angularis: Cell wall changes

The effects of auxin and gibberellic acid on cell wall composition in various regions of epicotyls of azuki bean ( Vigna angularis Ohwi and Ohashi cv. Takara) were investigated with the following results. (1) Young segments excised from apical regions of the epicotyl elongated in response to added 10⁻⁴ M indole-3-acetic acid (IAA). When the segments were supplied with 50 m M sucrose, the IAA-induced segment growth was accompanied by enhanced overall synthesis of cell wall polysaccharides, such as xyloglucans, polyuronides and cellulose. This IAA effect on the cell wall synthesis is a consequence of extension growth induced by IAA. Gibberellic acid (GA) at 10⁻⁴ M synergistically enhanced the IAA-induced cell wall synthesis as well as IAA-induced extension growth, although GA by itself neither stimulated the cell wall synthesis nor extension growth. In the absence of sucrose, cell wall synthesis was not induced by IAA or GA. (2) In mature segments excised from basal regions of the epicotyl, no extension growth was induced by IAA or GA. GA enhanced the synthesis of xylans and cellulose when the segments were supplied with 50 m M sucrose. IAA had no effect on the cell wall synthesis. These findings indicate that synthesis of polyuronides, xyloglucans and cellulose, which occurs during extension growth of the apical region of the epicotyl, is regulated chiefly by auxin whereas synthesis of xylans and cellulose during cell maturation in the basal region of the epicotyl is regulated by GA.